This invention relates to an integrated circuit of superconducting devices and, more particularly, to an integrated circuit consisting of plural superconducting circuit blocks and a method for designing the integrated circuit.
An integrated superconducting logic circuit comprises plural superconducting circuit blocks and superconducting wiring strips selectively connected between the superconducting circuit blocks. The superconducting circuit blocks carry out basic logical operations on input signals, and the output signals are propagated through the superconducting wiring strips. A circuit configuration of the simple rapid single flux quantum logic gates, i.e., RSFQ logic circuits and a connection between the superconducting circuits are disclosed by K. K. Kikharev et. al. in xe2x80x9cRSFQ Logic/Memory Family: A New Josephson-Junction Technology for Sub-Terahertz- Clock- Frequency Digital Systemsxe2x80x9d, IEEE Transactions on Applied Superconductivity, vol. 1, No. 1, pages 3 to 28, March 1991.
FIGS. 1A to 1C illustrate the current flowing in the prior art superconducting circuit blocks. In FIGS. 1A and 1B, superconducting circuit blocks 91 and 92 are connected to each other through a superconducting wiring strip 93, and the superconducting circuit blocks 95 and 96 are connected to each other through a superconducting wiring strip 97. The superconducting circuit block 91 is same in circuit configuration as the superconducting circuit block 95, and achieves a certain logic operation on input signals. The superconducting circuit block 92 is different in circuit configuration from the superconducting circuit block 96. However, it is not a problem that the superconducting circuit block 92 is same in circuit configuration as the superconducting circuit block 96.
When the superconducting circuit block 92 achieves the task, the superconducting circuit block 92 outputs static current 94 through the superconducting wiring strip 93 to the superconducting circuit block 91. This results in that the superconducting circuit block 91 is statically with the current more than the current flowing through the superconducting circuit block 91 in the isolated state due to the connection to the superconducting circuit block 92.
Similarly, the superconducting circuit block 95 supplies static current 98 through the superconducting wiring strip 97 to the superconducting circuit block 96. Accordingly, the superconducting circuit block 95 is statically supplied with current less than the current flowing through the superconducting circuit block 95 in the isolated state.
FIG. 1C illustrates the amount of current supplied to each of the superconducting circuit blocks 91 and 95. When the superconducting circuit blocks 91/95 are isolated, the operating range is represented by a block A. In order to give the maximum margin to the superconducting circuit blocks 91/95, the superconducting circuit blocks 91/95 are designed to have the amount of supply current at mid point A1 of the operating range A. When the superconducting circuit block 92 is connected to the superconducting circuit block 91, the amount of supply current is increased as described hereinbefore, and is moved to point A2. On the other hand, when the superconducting circuit block 95 is connected to the superconducting circuit block 96, the amount of current is decreased, and is moved to point A3.
Although point A2 is within the operating range, the margin is reduced. When the supply current is moved to point A3, which is out of the operating range A, the superconducting circuit block 95 becomes inoperative. The amount of supply current is regulable by changing design parameters of the superconducting circuit blocks. In order words, the supply current to each of the superconducting circuit blocks connected to one another is optimized at the mid point A1 by changing the design parameters for each superconducting circuit block. However, a large amount of time and labor is consumed in the optimization. Even though the superconducting circuit blocks such as those labeled with 91 and 95 are identical in circuit configuration with one another, the optimum set of design parameters is different between the superconducting circuit blocks 91 and 95, because the quantity and direction are different between the static current 94 and the static current 98. If a few superconducting circuit blocks form in combination an integrated circuit, the optimization will be not complicated. However, in case where a large number of superconducting circuit blocks are integrated into a complicated circuit, the optimization is a hard work.
Clark A. Hamilton et. al. report the optimization of parameters for the superconducting circuit blocks of a large scale integration to be difficult (see xe2x80x9cMargins and Yield in Single Flux Quantum Logicxe2x80x9d, IEEE Transactions on Applied Superconductivity, Vol. 1, No. 4, pages 157 to 163, December 1991). Thus, the prior art integrated circuit of the superconducting circuit blocks has a problem in that the optimization is difficult due to a large number of parameters to be considered in the optimization of the supply current.
This problem is inherent in the superconducting circuit, because it is difficult to separate an input signal from an output signal. On the contrary, input signals are separated from output signals in semiconductor large-scale integrated circuits, and the optimization work is not required for the semiconductor large-scale integrated circuits.
It is therefore an important object of the present invention to provide an integrated circuit of superconducting circuit blocks in which the amount of supply current to each superconducting circuit block is approximately equal to zero without changing parameters of the superconducting circuit block.
It is also an important object of the present invention to provide a method of designing an integrated circuit of the superconducting circuit blocks.
In accordance with one aspect of the present invention, there is provided a integrated circuit comprising a first superconducting circuit block including an output node, a first superconducting circuit for a certain function and a constant output circuit connected between an output node of the first superconducting circuit and the output node and making the amount of statically flow-in or flow-out current at an output node of the first superconducting circuit approximately equal to zero, and a second superconducting circuit block including an input node, a second superconducting circuit for a certain function and a constant input circuit connected between the input node and an input node of the second superconducting circuit and making the amount of statically flow-in or flow-out current at the input node of the second superconducting circuit approximately equal to zero.
By virtue of the constant output circuit and constant input circuit, the amount of statically flow-in and/or flow-out current is approximately equal to zero so that the superconducting circuit blocks are simply integrated without changing parameters of the superconducting circuits.
In accordance with another aspect of the present invention, there is provided a method of designing an integrated circuit of superconducting circuit blocks comprising the steps of a) determining a first superconducting circuit block having a constant output circuit, a second superconducting circuit block having a constant input circuit, an output current evaluation circuit and an input current evaluation circuit, b) determining parameters of the output current evaluation circuit and parameters of the input current evaluation circuit in such a manner that the amount of current is approximately equal to zero at an output node of the output current evaluation circuit connected to an input node of the input current evaluation circuit, c) separating the output current evaluation circuit from the input current evaluation circuit, d) connecting an input node of the output current evaluation circuit and an output node of the input current evaluation circuit to an output node of the constant output circuit and an input node of the constant input circuit, respectively, e) determining parameters of the constant output circuit and parameters of the constant input circuit in such a manner that the amounts of current are approximately equal to zero at the output node of the constant output circuit and at the input node of the constant input circuit, independently, f) disconnecting the output current evaluation circuit and the input current evaluation circuit from the constant output circuit and the constant input circuit, respectively and g) connecting the output node of the constant output circuit to the input node of the constant input circuit so as to integrate the first superconducting circuit block and the second superconducting circuit block together.